A turbine rotor constituting a turbocharger for a car comprises a turbine blade rotor made of a heat-resistant material and a rotor shaft made not necessarily of a heat-resistant material, which are separately manufactured from different materials and then joined together by e.g. electron beam welding, because the turbine blade rotor is required to be heat resistant. That is, as illustrated in FIG. 5, in a conventional turbine rotor 100, a joint face 106 of a turbine blade rotor 102 and a rotor shaft 104 is irradiated with electron beam E while both of the two are rotated, to weld the joint portion 106 over its entire circumference.
In this case, as the two components are circumferentially welded, a residual stress biased along the circumferential direction is generated in the welding portion. Accordingly, a gradient (leaning) 104′ of the rotor shaft indicated by the chain double-dashed line may arise, which leads to a shaft center leaning Δπ (see FIG. 5) at the circumference of the turbine blade rotor. Such a leaning may be corrected after welding by machine processing; however, the residual stress will not be removed thereby. Thus, during operation of the turbocharger, when the turbine rotor is heated, the residual stress generated at the time of welding is released, and the leaning of the turbine rotor 100 may occur again. There is a problem such that if leaning occurs, an unbalance force may be applied on the turbine rotor, which may lead to noise or vibration.
Patent Document 1 and 2 each discloses a method to weld a turbine blade rotor and a rotor shaft together by using electron beam. In the welding method disclosed in the Patent Document 1, while a turbine bade rotor made of a heat-resistant metal and a rotor shaft joined to each other are rotated, the joint portion is irradiated with electron beam to weld them together. Then, a prescribed width of the rotor shaft side from the joint portion is swept by and irradiated with electron beam to temper a portion of the rotor shaft near the joint portion, which has been hardened by the influence of the heat of the welding.
On the other hand, in the welding method disclosed in the Patent Document 2, electron beam irradiation devices are provided on plurality of points which are angularly-equally spaced around the joint portion of the turbine blade rotor and the rotor shaft, and welding are carried out at the same time at a number of points which are angularly-equally spaced. It is intended to thereby suppress occurrence of leaning of the rotor shaft due to a contraction stress generated along with solidification of molten metal at the time of welding.